I have a couple of students in my grade that are really great spellers and are keen to partake in a spelling bee to test out their ability.  I have found a few online sites for them to try to sharpen their skills that you might like to try with your own students.

Interactives Spelling bee has challenges for all year levels and is a great starting point.

The Times Spelling Bee Challenge is a bit of a step up in difficulty and will even challenge English Teachers.

Spelling Bee the Game - Based on the movie and is a great interactive challenge for the kids.  This is for your freakishly good spellers only.

Anyhow Good luck and let me know if you come across any others.